J.F.
I am a seasoned photographer, having worked years in a studio, here's my thoughts...
The baby would look best in a solid, crisp white onesie, and/or in only a diaper. There are also some amazing shots you can get if you're willing to have baby's little butt crack in the picture :) As for the older one, a nice polo shirt with horizontal stripes paired with kakhi shorts. Even, a solid polo is nice, if you prefer. I have always like the way little boys photograph in polos. Plain white, black or gray backgrounds will look the best as well.
Another idea (since you have 2 boys, you can do this no matter what their ages are) is to have the baby in only a diaper, and have big brother sans shirt. This makes an awesom "bust pose" if you can get big brother to hold baby super close. Let the photographer get creative and have fun and you'll be happy :)